1. Field of the Invention
The present invention relates to sensors for the detection of creatinine.
2. Description of Related Art
Continuous monitoring of creatinine can be accomplished by a number of electrochemical methods. Sensors utilizing such methods can be created by immobilizing the enzyme creatinine deiminase onto the surface of an electrode. The enzymatic hydrolysis of creatinine produces N-methylhydantoin and ammonia. At physiologic pH the ammonia is protonated to form ammonium ions, which increase the electrical conductivity of the solution proximal to the electrode.
Creatinine can also be monitored using an optical sensor. The detection of analytes by optical sensors usually requires the development of fluorescent transducers which are specific for different analytes. Optical transducers can also be coupled to the detection of creatinine via the creatinine deiminase driven hydrolysis of creatinine, with the optical transducer modulated by ammonium or ammonia.
Detection of ammonium requires an ammonium specific ionophore coupled to a chromophore that changes its absorption spectrum upon protonation, and a lipophilic anionic site. As such, sensors based on the detection of ammonium can be expensive and complex.
Detection of ammonia requires a protonated pH sensitive indicator (INDH.sup.+) which changes its absorption or fluorescence spectrum upon deprotonation: EQU INDH.sup.+ +NH.sub.3 .fwdarw.IND+NH.sub.4.sup.+
There is also a drawback to designing a sensor based on detection of ammonia: namely the rapid protonation of ammonia at physiologic pH. The pK.sub.a of ammonium is 9.3, which is not a pH that supports maximum enzyme activity.
Hydrophobic polymers, optically transparent and permeable to the analyte of interest, are used with optical sensors when the analyte is a vapor or gas and is capable of diffusion into a hydrophobic membrane. A complication arises when hydrophobic polymers are used with certain fluorescent dyes. Sensors for ammonia require a protonated indicator. When combined with a hydrophobic membrane for the detection of ammonia, polyanionic pH indicators, which are the common variety of protonated indicator and the type used in the fluorescent urea sensor described in Rhines and Arnold (Anal. Chim. Acta, 231: 231-235 (1990)), do not produce an activated and protonated fluorophore.
Sensors have been developed for the detection of creatinine based on enzymatic cleavage of creatinine. However, many of the sensors that have been developed for the detection of creatinine have been coupled to gas electrodes (Thompson and Rechnitz, Anal. Chem., 46: 246-249 (1974); Kihara and Yasukawa, Anal. Chim. Acta, 183: 75-80 (1986)), unlike the present invention, which couples the enzymatic cleavage of creatinine to detection by a fluorescent polymer coating.
While various indicators for creatinine are known, many sensors exhibit problems with interferences from pH and CO.sub.2 effects, low sensitivity, slow response times and reversibility. From a manufacturing standpoint, it would therefore be desirable to develop an inexpensive sensor capable of detecting creatinine that has a high sensitivity, fast response time, and is reversible. It would also be advantageous for the sensor to be able to function in conjunction with sensors detecting other analytes.